Dimerization through the catalytic domain is essential for MEKK2 activation

Mitogen-activated protein kinase (MAPK) cascades are the central components of the intracellular signaling networks that eukaryotic cells use to respond to a wide spectrum of extracellular stimuli. MAPKs are activated through a module consisting of a MAPK, a MAPK kinase (MKK), and a MKK kinase (MAP3K). Because of its unique position in the MAPK module, a MAP3K is crucial in relaying the upstream receptor-mediated signals through the MAPK cascades to induce physiological responses. Yet, the underlying molecular mechanism of MAP3K regulation and activation remains largely unknown. In this study, we demonstrated that MAP3K MEKK2 activation requires dimerization. We mapped the MEKK2 dimerization motif in its catalytic domain and showed that the NH2-terminal region is not required for MEKK2 dimer formation. We also found that the inactive, non-phosphorylated MEKK2 formed significantly more dimers than the phosphorylated and, hence, active MEKK2. Moreover, prevention of MEKK2 dimer formation inhibited MEKK2-mediated JNK activation. Using a chemical-induced dimerization system, we further demonstrated that MEKK2 dimer formation in vivo augmented MEKK2-dependent JNK activation and JNK/AP-1 reporter gene transcription. Together, these results suggest a novel mechanism underlying MEKK2 regulation and activation.